Background: Epidermal Langerhans cells (LCs) play a critical role in the induction of contact hypersensitivity. The LCs leave the skin, move to the regional lymph nodes and present the allergens embedded in the HLA–DR molecule to naive T–lymphocytes. To allow LC emigration from the epidermis, E–cadherin must be downregulated. In this study, we have examined the early events that occur in the human epidermis after exposure to three strong contact sensitizers and two commonly used fragrances by examining alterations of E–cadherin and HLA–DR expression. Methods: To determine whether E–cadherin and HLA–DR levels were modulated by allergens, flow cytometry was utilized to evaluate E–cadherin and HLA–DR expression on human epidermal LCs exposed to the different chemicals for 4 h at 37°C. Results: In vitro stimulation with the contact sensitizers isoeugenol, cinnamaldehyde, 2,4,6–trinitrobenzenesulfonic acid, Bandrowski’sbase, or p–phenylene diamine resulted in a dose–dependent decrease of HLA–DR expression on the surface of LCs without affecting the number of positive cells. These contact allergens induced a downregulation of E–cadherin expression as well as a significant decrease of the percentage of E–cadherin–positive cells. Incubation with an irritant, sodium lauryl sulfate, did not significantly change HLA–DR and E–cadherin expression. Conclusions: Based on the alteration of E–cadherin and HLA–DR expression of human LCs under short–term exposure conditions, there was a clear difference between contact sensitizers and a well–characterized irritant. For the first time, the ability of fragrance allergens in dipropylene glycol, a widely used vehicle in fragrance and cosmetic industries, was demonstrated to induce human LC phenotypic alterations. In combination with a series of in vitro tests, this rapid and simple method should help to detect the sensitizing potential of a substance to be applied onto the human skin as an alternative to animal testing.